26 – Distinguishing Between Aldehydes and Ketones (LC 2027) (Leaving Cert Chemistry): Revision Notes

26 – Distinguishing Between Aldehydes and Ketones

Introduction

Aldehydes and ketones are both carbonyl compounds, but they behave very differently in chemical reactions. The key difference lies in their ability to be oxidised - aldehydes can be easily oxidised to carboxylic acids, whilst ketones resist oxidation under mild conditions. This fundamental difference allows us to distinguish between these two functional groups using specific chemical tests.

We can identify aldehydes and ketones using three main tests:

• Fehling's reagent test
• Acidified potassium permanganate test
• Tollens' reagent test (silver mirror test)

Understanding the chemistry

Aldehydes are good reducing agents because they can donate electrons easily. When aldehydes are oxidised, they form carboxylic acids. For example, ethanal (an aldehyde) oxidises to ethanoic acid.

Ketones are much more resistant to oxidation because their carbon skeleton would need to be broken to achieve oxidation, requiring very strong oxidising conditions that would cause destructive changes to the molecule.

Test 1: Fehling's reagent test

What is Fehling's reagent?

Fehling's reagent is a mild oxidising agent made by mixing two solutions:

• Fehling's A: copper sulphate solution (blue colour)
• Fehling's B: potassium sodium tartrate and sodium hydroxide solution

When mixed, these create a royal blue solution containing $\text{Cu}^{2+}$ ions. The reagent must always be freshly prepared before use.

Results and observations

With aldehydes:

• A brick-red precipitate of copper(I) oxide forms
• The blue colour of the solution disappears
• This shows the aldehyde has reduced the $\text{Cu}^{2+}$ ions to $\text{Cu}^+$ ions

With ketones:

• No colour change occurs
• The solution remains blue
• No precipitate forms because ketones cannot reduce Fehling's reagent

Chemical explanation

The aldehyde acts as a reducing agent, causing the copper(II) ions to be reduced:

• $\text{Cu}^{2+} + e^- \rightarrow \text{Cu}^+$ (reduction - copper ions gain electrons)
• The aldehyde loses electrons and becomes oxidised to a carboxylic acid

Since ketones cannot be oxidised under these mild conditions, they show no reaction.

Test 2: Acidified potassium permanganate test

What is acidified potassium permanganate?

Acidified potassium permanganate ($\text{KMnO}_4/\text{H}^+$) is a strong oxidising agent with a distinctive purple colour. It contains permanganate ions ($\text{MnO}_4^-$) which can accept electrons from reducing agents.

Results and observations

With aldehydes:

• The purple colour disappears and becomes colourless
• This indicates the permanganate has been reduced by the aldehyde

With ketones:

• No colour change occurs
• The solution remains purple
• Ketones are not oxidised by this reagent under normal conditions

Chemical explanation

The aldehyde reduces the purple $\text{MnO}_4^-$ ions to colourless $\text{Mn}^{2+}$ ions:

$\text{MnO}_4^- + 8\text{H}^+ + 5e^- \rightarrow \text{Mn}^{2+} + 4\text{H}_2\text{O}$

The aldehyde is simultaneously oxidised to a carboxylic acid during this electron transfer.

Test 3: Tollens' reagent test (silver mirror test)

What is Tollens' reagent?

Tollens' reagent contains silver ions ($\text{Ag}^+$) in an alkaline solution. It's made by mixing silver nitrate solution with dilute sodium hydroxide and dilute ammonia. This reagent must be freshly prepared and used immediately.

Results and observations

With aldehydes:

• A shiny silver mirror forms on the inside of the test tube
• This distinctive mirror effect is the hallmark of a positive test
• The aldehyde has reduced silver ions to metallic silver

With ketones:

• No silver mirror forms
• No visible change occurs
• Ketones cannot reduce the silver ions

Chemical explanation

The aldehyde acts as a reducing agent, converting silver ions to metallic silver:

• $\text{Ag}^+ + e^- \rightarrow \text{Ag}$ (metallic silver deposits on glass)
• The aldehyde is oxidised to a carboxylate ion in the alkaline conditions

The metallic silver deposits as a thin, reflective layer on the glass surface, creating the characteristic mirror effect.

Key differences between the tests

Test	Oxidising Agent	Aldehyde Result	Ketone Result
Fehling's	$\text{Cu}^{2+}$ ions (mild)	Brick-red precipitate	No reaction
Acidified permanganate	$\text{MnO}_4^-$ ions (strong)	Purple → colourless	No reaction
Tollens'	$\text{Ag}^+$ ions (mild)	Silver mirror	No reaction

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